1. Field of the Invention
The present invention relates to a code-division multiplex communication apparatus for carrying out multiplex communication by using pseudo noise (PN) codes and to an application system thereof, and more particularly, to a code-division multiplex communication apparatus and an application system thereof for use in an environment wherein feeble radio waves are to be received or the strength of output radio waves is regulated.
2. Description of the Related Art
PN code-based communication apparatuses which carry out spread spectrum modulation using PN codes, are capable of multiplexing, and are tolerant to interference. Thus, this type of communication apparatuses is used for satellite communications dealing with feeble radio waves or in an environment wherein the strength of output radio waves is regulated.
A conventional code-division multiplex communication apparatus will be described below.
FIG. 1 is a block diagram illustrating a transmitting side of a conventional code-division multiplex communication apparatus wherein data channels used (hereinafter merely referred to as "channels") are three in number. The transmitting side comprises three exclusive-OR circuits 11 to 13 provided, respectively, for the data channels CH1 to CH3 to obtain exclusive ORs based on the data D1 to D3 and corresponding PN codes PN1 to PN3, double balanced modulators (DBM) 41 to 43 for subjecting the outputs X1 to X3 of the exclusive-OR circuits 11 to 13 to binary phase-shift keying (BPSK) by using a carrier wave f.sub.L, and a synthesizing circuit 60 for synthesizing the outputs of the modulators 41 to 43 and outputting the resulting signal as a transmission wave Sr.
The operation of the transmitting side constructed as above will be now explained.
FIG. 2 shows an example of PN codes used in the code-division multiplex communication apparatus. As illustrated, the PN codes used are the three-stage shift register-based M sequence codes (maximum length linear shift register sequence codes), and each has a maximum-length connection tap of [3, 1] and a code length of seven bits (per frame). Namely, the channels are three in number, and the PN codes associated with the channels are individually shifted by one bit. More specifically, the code PN2 of CH2 is shifted from the code PN1 of CH1 by one bit, and similarly, the code PN3 of CH3 is shifted from the code PN2 by one bit. The time length of this seven-bit PN code corresponds to the time length of one-bit data to be transmitted.
Now, the way of how data is transmitted will be described in detail.
FIG. 3 illustrates an example of how data is modulated using PN codes, wherein it is assumed that data D1 "1", data D2 "1" and data D3 "0" are to be transmitted through the channels CH1, CH2 and CH3, respectively.
First, exclusive-OR elements 11a, 12a and 13a each obtain an exclusive OR based on one-bit data and one-frame PN code associated with the corresponding channel. The signals outputted from the elements 11a, 12a and 13a are inverted by inverters 11b, 12b and 13b, respectively, thereby obtaining exclusive-OR outputs X1, X2 and X3. The inverters 11b, 12b and 13b are illustrated in order to simplify the following description only, and the strict exclusive-OR elements 11a, 12a and 13a and the corresponding inverters 11b, 12b and 13b constitute the exclusive-OR circuits 11, 12 and 13, respectively.
Phase modulation will be now described. FIG. 4 illustrates signal changes observed when the exclusive-OR outputs X1, X2 and X3 are subjected to binary phase-shift keying. The exclusive-OR outputs X1, X2 and X3 of the respective channels are subjected to binary phase-shift keying (BPSK) at double balanced modulators 41, 42 and 43 by means of the carrier wave f.sub.L on a time-slot basis. Specifically, the signal "1" is modulated to 0-degree phase, and the signal "0" is modulated to 180-degree phase, whereby phase-modulated signals S11, S12 and S13 are obtained. In FIG. 4, the shaded parts represent bits of 180-degree phase, and the unshaded parts represent bits of 0-degree phase.
Synthesis of the phase-modulated signals will be now described. FIG. 5 illustrates a signal change observed when the phase-modulated signals S11, S12 and S13 are synthesized by the synthesizing circuit 60. The phase-modulated signals S11, S12 and S13 of the three channels are synthesized on a time-slot basis (bit-by-bit basis) to obtain a composite signal S20. Namely, the amplitudes of in-phase signals are added up, while the amplitudes of signals having a phase difference of 180 degrees are canceled out. More specifically, since the three channels use the same carrier wave, the amplitude increases where in-phase signals overlap, and signals of opposite phases, when overlapped, cancel out each other. For example, three waves of the same 180-degree phase overlap at bit b.sub.1, and thus a composite wave having amplitude "3" is produced, whereas at bit b.sub.2, a 0-degree phase composite wave having amplitude "1" is produced because a wave of 180-degree phase overlaps with two waves of 0-degree phase.
FIG. 6 illustrates the polarity and amplitude of the composite signal. Namely, the waveform of the composite signal S20 is expressed by the polarity and amplitude of signal S30. In the graph showing the signal S30, "+1" represents a 0-degree phase wave having amplitude "1", and "+3" represents a 0-degree phase wave having amplitude "3". Similarly, "-1" represents a 180-degree phase wave having amplitude "1", and "-3" represents a 180-degree phase wave having amplitude "3".
Accordingly, the waveform of the transmission signal corresponding to one frame, i.e., EQU w=[b7, b6, b5, b4, b3, b2, b1]
can be expressed as
w=[1, 3, 1, -1, -1, 1, -3].
A transmission wave having such waveform is transmitted.
FIG. 7 is a block diagram schematically illustrating a receiving side of the code-division multiplex communication apparatus. The receiving side correlates the received signal Sr with each of the PN codes PN1 to PN3. First, the PN codes PN1 to PN3 are modulated at double balanced modulators 41b to 43b, respectively, by means of the carrier wave f.sub.L. Using the thus-modulated waves, the received signal Sr is again modulated by each of double balanced modulators 41a to 43a.
The modulated waves obtained in this manner are passed through band-pass filters (BPF) 101, 102 and 103 associated with the respective channels, whereby the results of the correlation corresponding to one frame of PN code are extracted in terms of amplitudes, based on the frequency band characteristics of the band-pass filters which are approximately twice the data rate. The outputs of the filters are passed through PSK demodulators 111 to 113, respectively, thereby demodulating the data D1 to D3.
Code synchronization and carrier-wave synchronization between the receiving and transmitting sides are carried out at the receiving side by methods known in the art. Thus, illustration and description of code synchronization circuit and carrier-wave synchronization circuit are omitted.
Whether the data can be accurately demodulated depends on the results of the correlation.
FIG. 8 illustrates PN codes used for the demodulation, in terms of polarity and amplitude. PN codes PN1 to PN3 associated with the respective channels are identical with the corresponding PN codes used at the transmitting side. Each of the PN codes is subjected to the binary phase-shift keying such that 0-degree phase and 180-degree phase are represented by "1" (unshaded bit in the figure) and "0" (shaded bit), respectively, thus obtaining signals S41 to S43. Further, the 0-degree phase and the 180-degree phase are expressed as "1" and "-1", respectively, whereby signals S51 to S53 are obtained.
Then, the signals S51 to S53 shown in FIG. 8 are each correlated with the signal S30 shown in FIG. 6. This correlation and integration are equivalent to obtaining the inner product of the transmission waveform S30 and each of the PN codes S51 to S53. The value of each inner product represents the amplitude of corresponding demodulated data. The following shows actual correlation with regard to CH1 to CH3: ##EQU1##
Provided that data is "1" if the result of the correlation indicates a positive (+) number, and that data is "0" if the result of the correlation indicates a negative (-) number, then the data of CH1, CH2 and CH3 are "1", "1" and "0", respectively. Thus, the transmitted data can be demodulated.
In the case where the signals of the respective channels are transmitted independently without being multiplexed, and then demodulated, the values of demodulation correlation are +7, +7 and -7, respectively.
Thus, when the composite waveform is transmitted faithfully, the receiving side can acquire demodulated levels that are almost identical to those which would be obtained when the signals are not multiplexed, as long as the PN codes used at the transmitting side are known.
In the above-described code-division multiplex communication apparatus, however, the amplitude, i.e., the electric field strength, of the transmission wave increases in proportion to the number of channels used for the transmission, as seen from FIG. 5. For example, in the aforementioned example, the number of the channels is three, and thus the amplitude is increased by three times.
If, on the other hand, the electric field strength of the transmission wave is to be fixed, the electric field strength per channel must be 1/3 where the number of channels used is three, which leads to a lowering of the SN (signal-to-noise) ratio. Thus, in situations where the electric field strength of a transmission wave is regulated, the SN ratio lowers as the number of channels is increased.
Generally, the electric field strength of the transmission wave is legally regulated, and there is a demand for a transmission apparatus which has as high SN ratio within the regulated range as possible.
Further, the transmission apparatus should preferably be simplified in a circuit arrangement thereof, and there is a particular demand for a transmission apparatus which has high SN ratio and can be used in a radio LAN or FA (factory automation) system dealing with feeble radio waves and installed in an environment involving many noise sources.